Both the simplest and hardest answer to your question: Try to prove them right first.
This is probably best explained by giving a concrete example. Let's assume you're debating about what color a trafic light should have. You guys are convinced it should be red/yellow/green right? Since those colors are legaly enforced. But then there's this one guy that just really wants it to be red/yellow/blue instead because it's easier to see.
It's impossible to convince him about green being the only correct answer "because the law says so".
Instead let's first actually try to understand his point of view. Why does he think blue is easier to differenciate than green? By first actally trying to understand why he thinks so we discover he's colorblind and has trouble with green/red.
Hey, this is actually a really valid reason to make it blue! If only the law allowed for it.
It makes a huge difference to respond to him with "you're wrong, it HAS to be green" and "I agree with you that blue would be a good way, but it's not legal so we can't do that".
The end result might be something like these japanese trafic lights which are the blue-est possible color that is still officially green.
By first trying to prove them right you force yourself to actually listen and understand their point of view. At least in this step it will prevent them from going defensive.
The next step is asking them (nicely!) to do the same for your point of view. Have them try to see the possitive parts of your side.
Only when you find out where your standpoints actually differ you can start finding a solution. Or - depending on the reason for the debate - you can agree on having different opinions and leave it at that.
A final thing to really think about: What makes you so sure the 5 of you were really right in the first place?
(Rhetorical question, please don't try to answer this in a comment.)